(Circulation. 1998;98:1228-1235.)
© 1998 American Heart Association, Inc.
Role of Adenosine Receptors in the Paradoxic Bradycardia Response of Rats to Inferior Vena Cava Occlusion During an Infusion of Isoproterenol
Menashe B. Waxman, MD, FRCP(C);
; John A. Asta
From the Department of Medicine of the University of Toronto and the
Division of Cardiology of the Toronto Hospital, Ontario, Canada.
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Abstract
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Background—In susceptible humans,
vasodepressor reactions are induced by restriction of venous return
(upright tilting) and administration of isoproterenol. Because
paradoxic bradycardia is a major manifestation of vasodepressor
reactions, and allowing for extrapolation between paradoxic bradycardia
in rats and vasodepressor reactions, we examined whether
adenosine receptors mediate the paradoxic bradycardia
reaction.
Methods and Results—Paradoxic bradycardia was induced in rats by
inferior vena cava occlusion during an isoproterenol
infusion. We studied whether dipyridamole, an
adenosine transport inhibitor, and aminophylline
(nonselective) or DPCPX (selective) A1
antagonists augmented or inhibited paradoxic bradycardia,
respectively, during inferior vena cava occlusion. The
maximum changes in R-R during 60 seconds of inferior vena
cava occlusion were that (1) in control, the rate accelerated (
R-R,
-9.7±0.8 ms, P<0.001); (2) during isoproterenol (0.8
µg · min-1), paradoxic bradycardia occurred
(R-R, +92.0±32.0 ms, P<0.001); (3) during
isoproterenol but after dipyridamole, paradoxic
bradycardia occurred at a much lower dose of isoproterenol (0.2
µg · min-1), and the magnitude was increased at
all doses (at 0.8 µg · min-1 isoproterenol,
R-R, +195.6±27.6 ms, P<0.001 versus isoproterenol
alone, R-R, +92.0±32 ms); (4) during isoproterenol and
dipyridamole, atropine did not block paradoxic
bradycardia, but cervical vagotomy inhibited paradoxic bradycardia
(R-R, +5.6±1.8 ms, P<0.001 compared with
isoproterenol and dipyridamole alone); and (5) during
isoproterenol alone, aminophylline or DPCPX blocked paradoxic
bradycardia (R-R, -5.4±1.0 ms, and R-R, -2.6±0.5 ms,
respectively, each P<0.001 compared with isoproterenol
alone).
Conclusions—The adenosine A1 receptor
mediates the paradoxic bradycardia reflex during inferior
vena cava occlusion in the face of isoproterenol via vagal afferents.
Key Words: heart rate • adenosine • receptors • reflex
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Introduction
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Despite
extensive research, our understanding of the mechanism and the therapy
of vasodepressor reactions remains limited.1
Vasodepressor reactions occur in response to pain, fright, orthostasis,
blood loss, myocardial ischemia, vasodilator drugs, or after
extreme exercise.1 2 3 Reactions also occur in
fighter pilots and motor vehicle drivers, and fatal crashes can
ensue.4 Thus, vasodepressor reactions are
important because they occur in response to a wide variety of stimuli
and they can lead to hypotension, cardiac asystole, syncope, injury, or
even death. Development of effective treatment requires an
understanding of the triggering mechanism.
We developed a clinical test to assess human susceptibility to the
vasodepressor reaction and its treatment.5
Isoproterenol administered during restricted venous return (+60°
tilt) provokes paradoxic bradycardia and hypotension in persons prone
to such reactions. Subsequently, we developed a related model in the
rat consisting of inferior vena cava occlusion combined
with an infusion of isoproterenol.6 7 8 9
Inferior vena cava occlusion causes marked hypotension, and
without isoproterenol, it increases heart rate through reflexes. When
isoproterenol is administered, however, inferior vena cava
occlusion causes paradoxic bradycardia. Bradycardia during hypotension
is a major manifestation of clinical1 2 and
experimental1 10 vasodepressor reactions.
Although our previous work defined some of the neural pathways involved
in reflex bradycardia,6 7 8 9 we have not examined
whether endogenous chemicals play a role in initiating the
reaction. We believe that adenosine may play a central role in
the vasodepressor reaction, for the following reasons: (1)
adenosine, an endogenous purine, is elaborated
during low blood flow11 and high adrenergic
drive,12 conditions that are present in our
model; (2) ischemia13 and
adenosine14 can excite vagal afferents;
(3) paradoxic bradycardia reaction in our model depends on cardiac
vagal afferents6 7 8 9 ; (4) exogenously administered
adenosine during upright tilting can elicit a vasovagal
reaction in humans15 ; and (5) in 2 reports,
patients have been successfully treated for vasodepressor reactions
with theophylline.16 17
We investigated whether adenosine participates in paradoxic
bradycardia in our rat model. We measured the paradoxic bradycardia
response to inferior vena cava occlusion and isoproterenol
without and after pretreatment with
dipyridamole,18 an
adenosine transport inhibitor, and
aminophylline,19 a nonselective, as well as
1,3-dipropyl-8-cyclopentyl- xanthine (DPCPX), a selective
A1 adenosine receptor
antagonist.20
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Methods
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Protocol
A total of 35 adult male Wistar rats (360.2±5.8 g) were used in
these studies; the methods have been extensively described
previously.6 7 8 9 The animals were
anesthetized by intraperitoneal inactin,
paralyzed with intravenous pancuronium, and ventilated at
70 breaths per minute through a tracheostomy (Harvard
Apparatus, model 680). In preliminary experiments, it was
shown that inactin completely inhibited any withdrawal reflex in
response to pressure applied to the extremities for the entire
experimental period before pancuronium was used. The animal's core
temperature was kept at 37±0.5°C. A Silastic cannula inserted into
the carotid artery was attached to a transducer (Statham, P23db) for
blood pressure measurement. Another Silastic cannula inserted in the
jugular vein was used for fluid and drug administration. Through a low
sternotomy, a thin umbilical tape was passed around the
inferior vena cava just as it enters the chest for periodic
occlusion. Under the conditions of the protocols described below, the
inferior vena cava was occluded for 60 seconds twice during
any particular intervention or drug dosage.
Inferior Vena Cava Occlusion During Administration
of Isoproterenol
Six rats were studied before and during isoproterenol infusion.
The inferior vena cava was occluded for 60 seconds during
control conditions. Isoproterenol was then infused starting at 0.1
µg · min-1 and sequentially doubled
until we observed R-R interval prolongation (
20 ms) during 60 seconds
of inferior vena cava occlusion to a concentration of 0.8
µg · min-1. If R-R prolongation was not
observed, the dose of isoproterenol was increased to a maximum of 1.0
µg · min-1, at which dose R-R
prolongation occurred.
Inferior Vena Cava Occlusion During Administration of
Isoproterenol in Rats Pretreated With Dipyridamole
Before and After Treatment With Atropine and Bilateral Cervical
Vagotomy
Nine rats treated with dipyridamole 2.0 mg were
studied before and during isoproterenol. The inferior vena
cava was occluded for 60 seconds during control conditions.
Isoproterenol was infused starting at 0.1 µg ·
min-1 and sequentially doubled to a maximum
concentration of 1.0 µg · min-1 (as
above), and at each step the inferior vena cava was
occluded. In separate experiments, dipyridamole (2 mg)
caused a 4-fold increase in the R-R interval prolongation in response
to adenosine (20 to 80 µg) (see Results).
Another group of 5 rats were pretreated with 2 mg
dipyridamole, and the inferior vena cava
was occluded during an infusion of isoproterenol 0.4 µg ·
min-1. The dose chosen was insufficient to cause
paradoxic bradycardia by itself, but when combined with
dipyridamole, it resulted in paradoxic bradycardia
equal to or greater than that seen in response to 0.8 µg ·
min-1 isoproterenol without
dipyridamole. The experiments were repeated after
pretreatment with atropine 1 mg IP and after bilateral cervical
vagotomy. The bradycardia response to electrical stimulation of the
sectioned efferent cervical vagus nerves was blocked in each rat after
pretreatment with 1 mg atropine IP.
Inferior Vena Cava Occlusion During Administration of
Isoproterenol After Treatment With Aminophylline or DPCPX
Ten rats were studied before and during an infusion of
isoproterenol. The inferior vena cava was occluded for 60
seconds during control, after which isoproterenol was infused starting
at 0.25 µg · min-1, and this was
sequentially doubled to a maximum of 1.0 µg ·
min-1 or until we observed R-R interval
prolongation (20 ms) during inferior vena cava occlusion.
With infusion of isoproterenol maintained, the rats received
aminophylline as a 0.5-mg bolus, and this was sequentially doubled to a
maximum of 4.0 mg or until inhibition of R-R interval prolongation
during inferior vena cava occlusion was achieved (mean
dose, 2.5±0.3; range, 1 to 4 mg). In separate experiments,
aminophylline (2 mg) caused a 3.44-fold decrease in the R-R interval
prolongation after administered adenosine (20 to 80 µg) (see
Results).
Five additional rats were studied after DPCPX treatment. After the dose
of isoproterenol that resulted in paradoxic bradycardia during
inferior vena cava occlusion had been established, DPCPX
was administered as a bolus starting at 25 µg, and this was
sequentially doubled up to a maximum of 200 µg or until paradoxic
bradycardia was blocked (150±25 µg). In separate experiments, DPCPX
(150 µg) caused a 3.8-fold reduction in the R-R interval prolongation
in response to adenosine (20 to 80 µg) (see Results).
Preliminary Experiments to Establish Effects of Aminophylline,
Dipyridamole, and DPCPX on Response of Heart Rate to
Exogenously Administered Adenosine
In a separate group of 9 closed-chest anesthetized rats,
we administered adenosine 20, 40, and 80 µg IV and measured
the R-R interval change in control and after
dipyridamole (0.5 to 2 mg), aminophylline (0.5 to 4
mg), and DPCPX (25 to 200 µg).
Data Analysis and Statistical Analysis
ECG and blood pressure signals were digitized and sampled
continuously every 20 ms and after 50 samples, were stored as 1-second
values by a microcomputer system.6 7 8 9 The
1-second samples were averaged into 5-second segments covering the
control period (20 seconds), the inferior vena cava
occlusion (60 seconds), and after release (20 seconds). The data during
the 20 seconds of control were averaged into a single value for the
resting R-R interval and blood pressure. The maximum change (increase
or decrease) during the 60-second period of inferior vena
cava occlusion was measured.
All data were expressed as mean±SEM. The blood pressure response to
drug administration and the R-R interval during resting conditions in
each group of experiments were analyzed with Student's
t test for paired data. A 1-way ANOVA for repeated
measurements was used when a comparison involved >2 states and was
followed by post hoc testing with Dunnett's test to identify the
significant comparisons. Linear regression analyses
calculated the relationship between the dose of administered
isoproterenol and the change in R-R interval before and after
dipyridamole. A 1-way ANOVA compared the maximum blood
pressure fall during inferior vena cava occlusion in the
different states. A probability value of <0.05 was considered
significant.21
The methods used in these experiments conformed to the guidelines on
animal use of the American Heart Association (1984). The protocols were
reviewed and approved by the Animal Care Committee of the University of
Toronto.
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Results
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Effects of Aminophylline, Dipyridamole, and DPCPX
on Response of Heart Rate to Exogenously Administered
Adenosine
There was a dose-dependent prolongation in the R-R interval
after the administration of adenosine. The effect of
adenosine was enhanced by dipyridamole and
blocked by aminophylline and DPCPX. Plots of the log R-R versus log
doses of adenosine under control, dipyridamole,
aminophylline, and DPCPX were obtained. Dose ratios were then
calculated for the doses of dipyridamole,
aminophylline, and DPCPX that approximated the doses of these
adenosine agonist/antagonists that were closest to
the average dose in the experiments that enhanced or blocked the R-R
response to inferior vena cava occlusion + isoproterenol.
There was a 4-fold increase in the response to adenosine in the
presence of dipyridamole (2 mg) and 3.44- and 3.8-fold
reductions in response to adenosine after aminophylline (2 mg)
and DPCPX (150 µg), respectively.
Effects of Isoproterenol on Resting Blood Pressure and Effects of
Inferior Vena Cava Occlusion on Blood Pressure and
Pulse Pressure
During an infusion of isoproterenol (0.8±0.13 µg ·
min-1), the mean blood pressure fell from
122.7±6.2 to 91.3±8.1 mm Hg (n=30 rats, P<0.001,
paired Student's t test), and the pulse pressure increased
from 29.2±2.2 to 64.6±2.7 mm Hg (P<0.001, paired
Student's t test). During inferior vena cava
occlusion in all the different states, the mean blood pressure fell to
a minimum of 17.3±5.3 mm Hg (range, 14 to 27 mm Hg,
P=NS, ANOVA). During inferior vena cava
occlusion, the pulse pressure fell to 11.4±1.3 and 12.7±0.64
mm Hg during control and isoproterenol, respectively (P=NS,
ANOVA).
Inferior Vena Cava Occlusion During Graded Infusion
of Isoproterenol
Without isoproterenol, inferior vena cava occlusion
shortened the R-R interval (-9.7±0.8 ms, paired Student's
t test, P<0.001). During an infusion of a lower
amount of isoproterenol (0.1 to 0.4 µg ·
min-1), inferior vena cava occlusion
produced significant shortening of the R-R interval (-13.9±1.1,
-15.7±1.4, and -7.2±1.1 ms, respectively, P=NS compared
with control). During higher amounts of isoproterenol (0.8 and 1.0
µg · min-1), inferior vena
cava occlusion caused paradoxical R-R interval prolongation
(+92.0±32.0 and +118.8±40.5 ms, respectively, P<0.001
compared with control; Table
, row 2, columns 1 through 6; ANOVA).
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Table 1. Inferior Vena Cava Occlusion During Graded
Infusion of Isoproterenol Without (Top) and After Pretreatment
With Dipyridamole (Bottom)
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Inferior Vena Cava Occlusion During Administration of
Isoproterenol in Rats Pretreated With Dipyridamole
After dipyridamole treatment, inferior
vena cava occlusion without isoproterenol prolonged the R-R interval
minimally (+5.2±2.6 ms). During isoproterenol alone (without
dipyridamole), inferior vena cava occlusion
did not cause paradoxic bradycardia at doses of isoproterenol <0.8
µg · min-1. After
dipyridamole, however, inferior vena cava
occlusion resulted in significant R-R prolongation at isoproterenol
doses of 0.2 µg · min-1, and the R-R
prolongation during inferior vena cava occlusion after
dipyridamole pretreatment was significantly higher than
with isoproterenol alone (Figure 1 and
Table
, row 4, columns 1 through 6).
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Figure 1. Plot of maximum R-R response to 60 seconds of
inferior vena cava occlusion during graded doses of
isoproterenol without and after dipyridamole.
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Linear regression analysis showed a significant increase in the
change in R-R in response to isoproterenol [R-R interval=163.3(dose
of isoproterenol)-46.9; r=0.94, P<0.01].
Similarly, linear regression analysis also showed a significant
dose-dependent increase in the changes in R-R in the group
pretreated with dipyridamole [R-R
interval=184.7(dose of isoproterenol)+26.7; r=0.75,
P<0.01]. There was a significant shift in the slope of the
2 dose-response curves, showing a difference of +75.1 ms
(P<0.01).
Inferior Vena Cava Occlusion During Administration of
Isoproterenol in Rats Pretreated With Dipyridamole
After Treatment With Atropine and After Bilateral Cervical
Vagotomy
In 5 separate experiments, rats were pretreated with
dipyridamole. An infusion of isoproterenol (0.4
µg · min-1) was administered. In
untreated rats, this dose was insufficient to provoke paradoxic
bradycardia during inferior vena cava occlusion, but after
treatment with dipyridamole (see above), this dose
caused paradoxic bradycardia equal to or greater than that seen in
response to 0.8 µg · min-1 of
isoproterenol. After treatment with atropine, the R-R interval
prolongation during inferior vena cava occlusion was
unchanged (+126.2±22.3 ms, P=NS compared with isoproterenol
+ dipyridamole). Bilateral cervical vagotomy blocked
the paradoxic bradycardia response to inferior vena cava
occlusion (R-R, -5.6±1.8 ms, P<0.001 compared with
isoproterenol + dipyridamole with intact vagus nerves
by ANOVA).
Inferior Vena Cava Occlusion During Administration of
Isoproterenol Before and After Treatment With Aminophylline or
DPCPX
Without isoproterenol, inferior vena cava occlusion
shortened the R-R interval (-13.2±2.1 ms, P<0.001 by
paired Student's t test). During an infusion of
isoproterenol (0.8 µg · min-1),
inferior vena cava occlusion significantly prolonged the
R-R interval (+108.9±13.0 ms, P<0.001, ANOVA).
Aminophylline (2.5±0.3 mg; range, 1 to 4 mg) blocked the R-R
prolongation during isoproterenol and inferior vena cava
occlusion (-5.4±1.0 ms, P<0.001, ANOVA; see Figure 2).
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Figure 2. Plot of R-R interval every 5 seconds before,
during, and after inferior vena cava occlusion in control
and during infusion of isoproterenol before and after aminophylline
treatment. Paradoxic bradycardia during inferior vena cava
occlusion is blocked by aminophylline.
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In 5 additional rats, the paradoxic bradycardia response to
isoproterenol (0.8 µg · min-1) was
established (R-R, +132.4±20.4 ms, P<0.001 compared with
control). DPCPX blocked paradoxic bradycardia (R-R, -2.6±0.5 ms,
P<0.001 compared with isoproterenol, P=NS
compared with control; Figure 3).
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Figure 3. Plot of R-R interval every 5 seconds before,
during, and after inferior vena cava occlusion in control
and during an infusion of isoproterenol before and after DPCPX
treatment. Paradoxic bradycardia during inferior vena cava
occlusion is blocked by pretreatment with DPCPX.
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Summary of Responses to Inferior Vena Cava
Occlusion
A graphic summary of the maximum changes in R-R interval during
inferior vena cava occlusion in 10 states (columns 1
through 10) is shown in Figure 4. Section
1 is recorded after pretreatment with dipyridamole.
The R-R interval shortened during control conditions (column 1); the
R-R prolonged during an infusion of a subthreshold dose, 0.4 µg
· min-1 of isoproterenol (column 2); paradoxic
bradycardia was not inhibited by atropine (column 3); and paradoxic
bradycardia was blocked by vagotomy (column 4). Section 2 and 3 were
recorded without dipyridamole treatment. The R-R
interval shortened during control conditions (columns 5 and 8); the R-R
prolonged during an infusion of isoproterenol (columns 6 and 9); and
the R-R interval prolongation during an infusion of isoproterenol was
blocked by either aminophylline (column 7) or DPCPX (column 10).
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Figure 4. Graphic summary of maximum changes in R-R interval
during inferior vena cava occlusion in 10 states (columns 1
through 10). Columns 1 through 4 were recorded after pretreatment
with dipyridamole. Columns 5 through 10 were
recorded without dipyridamole treatment (see
Results).
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Discussion
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In our model, there is low blood flow because of
inferior vena cava occlusion as well as intense adrenergic
stimulation caused by the infusion of isoproterenol and reflex
activation of sympathetic tone.6 7 8 9 Because
adenosine is elaborated in response to such
conditions,11 12 we examined the role of
adenosine receptors on the paradoxic bradycardia reflex in this
model. Two aspects of this study show that adenosine receptors
mediate the paradoxic bradycardia response to inferior vena
cava occlusion and isoproterenol. First, dipyridamole
blocks the uptake of adenosine into cells, thereby raising the
extracellular adenosine concentration,18
and this allowed paradoxic bradycardia to occur at a much lower dose of
isoproterenol and enhanced paradoxic bradycardia in response to
inferior vena cava occlusion during an infusion of
isoproterenol. Second, aminophylline,19 a
nonselective, and DPCPX,20 a selective
A1 adenosine receptor
antagonist inhibited the paradoxic bradycardia response to
inferior vena cava occlusion during an infusion of
isoproterenol. In this regard, 2 recent publications claim success in
treating humans for vasodepressor reactions with
theophylline.16 17
Earlier studies showed that the paradoxic bradycardia in this rat model
depends on cardiac vagal afferents: it is blocked by cervical vagotomy
or intrapericardial lidocaine but is independent of efferent vagal tone
because it is unaffected by muscarinic receptor blockade with
atropine.6 7 Similarly, the paradoxic bradycardia
response to inferior vena cava occlusion during
isoproterenol and dipyridamole was not affected by
pretreatment with atropine, but it was blocked by cervical vagotomy.
Thus, paradoxic bradycardia is independent of muscarinic receptors but
depends on vagal afferents. To demonstrate this, we administered a dose
of isoproterenol (0.4 µg · min-1) that
was insufficient to cause paradoxic bradycardia during
inferior vena cava occlusion. However, when rats were
pretreated with dipyridamole, this dose of
isoproterenol resulted in paradoxic bradycardia equal to or greater
than that seen with 0.8 µg · min-1
isoproterenol. Thus, we set up conditions in which both
dipyridamole and isoproterenol were essential in
producing paradoxic bradycardia.
Because it has been suggested that anomalous firing of carotid sinus
baroreceptors might be the afferent source of vasodepressor reactions
experimentally and in humans,22 we considered
this possibility in our experiment. In this regard, it is notable that
isoproterenol markedly increased the pulse pressure, and this can
exaggerate afferent firing from the baroreceptors at any level of mean
arterial pressure.23 However, an
augmented pulse pressure is unlikely to be responsible for paradoxic
bradycardia, because the pulse pressure during inferior
vena cava occlusion with or without isoproterenol fell to the same
levels. Furthermore, without inferior vena cava occlusion,
isoproterenol augmented the pulse pressure, and yet paradoxic
bradycardia was not seen. In addition, in recent experiments we
demonstrated that paradoxic bradycardia in this model was unaffected by
carotid sinus nerve section.24
Previous work showed that the bradycardia is secondary to a reflex
withdrawal sympathetic tone, because bilateral stellate ganglionectomy
or chemical sympathectomy
(6-hydroxydopamine)6 7 or the
administration of propranolol8 slows
the heart rate to values that occur during inferior vena
cava occlusion, and after these interventions, the bradycardia during
inferior vena cava occlusion is inhibited. In a related
experiment involving inferior vena cava occlusion in the
rabbit, bradycardia was due to
sympathoinhibition.25 By contrast, bradycardia in
human vasodepressor reactions is usually due to increased efferent
vagal tone.1 5
Adenosine modulates and regulates cell metabolism
through specific surface receptors A1 and
A2 and through intracellular binding
sites.26 Apart from coronary
vasodilatation, which is regulated by A2
receptors, all other cardiac adenosine actions are mediated via
A1 receptors.19 26 During
decreased oxygen supply (ischemia)11 or
accelerated ATP usage (excessive catecholamine
drive),12 adenosine production
rises. By decreasing metabolism and by causing
vasodilatation to increase blood flow and oxygen delivery during
increased metabolic activity and during ischemia,
adenosine protects the heart during
ischemia.27 Adenosine
production correlates with the extent of
ischemia.11
The mechanism whereby inferior vena cava occlusion and
isoproterenol trigger the reflex has not been established. It is
believed that an increase in left ventricular
contractility is salient to triggering the
vasodepressor reaction clinically and
experimentally.28 In this regard, we recently
showed that pretreatment with verapamil inhibited paradoxic
bradycardia,29 thereby offering support to the
hypothesis that augmented contractility is part of the
triggering mechanism.28 In further support of the
concept that an increase in contractility is needed to
trigger the reflex, we found that if we prevent a rise in sympathetic
tone during the initial phase of inferior vena cava
occlusion by maintaining the carotid sinuses at normal pressure, the
paradoxic bradycardia reflex is inhibited.24
Adenosine interacts with adrenergic neurons at prejunctional
and postjunctional sites by altering catecholamine release
and by directly altering the effects of catecholamines at
the receptor level.26 Adenosine inhibits
norepinephrine release directly via purinergic receptors in
open-chest dogs.30 Furthermore, adenosine
inhibits neurotransmission in sympathetic ganglia, leading to
inhibition of norepinephrine release from efferent
sympathetic nerves.31 The main actions of
catecholamines on the myocardium are mediated
by cAMP.26 ß-Adrenergic stimulation augments
adenosine levels, which work through A1
receptors to decrease cAMP and act as a natural feedback
inhibitor of catecholamines in the
myocardium.32 Ultimately,
adenosine attenuates the adrenergically stimulated rise in cAMP
and the increase in L-type calcium inward current
(ICa,L) in atrial and
ventricular tissue.32
Adenosine working through prejunctional and postjunctional
mechanisms could reduce the release of norepinephrine as
well as the cellular response to adrenergic tone. Thus,
adenosine could act on the efferent side of the reflex, where
its actions would be additive with the reflex sympathoinhibition that
is responsible for paradoxic bradycardia in our
model.6 7 This could explain the augmented
bradycardia after dipyridamole.
Adenosine-mediated sympathoinhibition could also act on the
afferent side of the reflex. Previously, we showed that paradoxic
bradycardia required a reflex rise in sympathetic tone to trigger the
afferent mechanism of the reaction.24 Thus,
adenosine, by its ability to reduce sympathetic
transmission,31 release of
norepinephrine,30 and the
intracellular effects of adrenergic
stimulation,26 32 could block the reaction.
However, such an action would not fit with the observed results,
because dipyridamole enhanced rather than blocked the
reaction.
A further effect of adenosine on paradoxic bradycardia is
possible. In a recent publication in which adenosine elicited
vasovagal reactions in humans undergoing upright tilting, it was
postulated that adenosine works by causing sympathetic
activation.15 Because an increase in sympathetic
tone is essential for the paradoxic bradycardia reflex in our
model,24 adenosine may initially act at
this level to trigger the reflex. In summary, adenosine may act
by sympathetic excitation to initiate the reflex, and once started,
adenosine may also augment the sympathoinhibition on the
efferent side that is responsible for paradoxic bradycardia.
Previously, we showed that phenylephrine inhibits the
paradoxic bradycardia response to inferior vena cava
occlusion, whereas prazosin facilitated paradoxic
bradycardia.9
1-Adrenergic stimulation and
1-blockade augment and decrease the release of
adenosine from ischemic myocardium,
respectively.33 Furthermore,
1-adrenergic blockade with prazosin decreases
adenosine release from ischemic myocardium
and decreases coronary blood flow and reduces reactive
hyperemia.33 The finding that
-stimulation raises adenosine levels, which in turn
counteracts the effects of ischemia,33
and the fact that phenylephrine, an
1-agonist, blocks paradoxic bradycardia in our
model, whereas phentolamine and prazosin,
1-receptor antagonists, facilitate
the reaction,9 lead us to believe that
phenylephrine and prazosin are not working through
adenosine in our model. Alternatively, the conditions of our
experiments may not be applicable to some models of experimental
ischemia.
Adenosine and a related purine ATP slow the sinus rate in
humans, animals, and isolated tissues by
hyperpolarization via a time-independent potassium
current (IKado) and a reduced rate of
diastolic depolarization.26 As in our
rat model, in which the bradycardia is independent of
atropine,6 7 the bradycardia induced by
adenosine given systemically or into the sinoatrial node is
unaffected by atropine in dogs.34 35 36 37 The
bradycardia caused by adenosine or ATP and the bradycardia in
our experiment are blocked by aminophylline, an
A1-receptor
antagonist.37 However, unlike our rat
model, in which vagotomy blocks bradycardia,8 9
the bradycardia after exogenous adenosine is not blocked by
vagotomy in dogs34 or
cats.38
ATP is rapidly hydrolyzed to adenosine, and when a
nonhydrolyzable ATP is injected into the sinus node
artery, there is no bradycardia.37 Thus, the
action of ATP on the sinus node is probably via ATP conversion to
adenosine, and the latter acts on A1
receptors to slow the rate.26 In addition to a
direct effect on the sinus node, ATP also induces reflex slowing via
cardiac vagal afferents in cats and dogs34 39 but
not in rabbits.38 39 Thus, ATP causes bradycardia
by direct effects on the sinus node as well as by vagally mediated
reflex–induced slowing. Like the bradycardic effects of ATP, which
require conversion of ATP to adenosine when it is injected
directly into the sinus node artery, the reflex stimulation of vagal
afferents by ATP may also work through conversion to adenosine.
According to this reasoning, endogenously formed
adenosine may trigger vagal afferents. Thus, not only may ATP
induce a vagal afferent bradycardic reflex via adenosine, but
it is also possible that when adenosine is released
endogenously as a result of ischemia, etc, the
ensuing bradycardia may involve a vagal afferent reflex. Thus, released
adenosine could work directly on the sinoatrial node to slow
the rate, or it may slow the rate by stimulating vagal afferents in our
experiment that cause reflex sympathoinhibition and bradycardia.
Experimental and clinical evidence suggests that adenosine
release during ischemia might be responsible for cardiac
pain.40 Experimentally induced myocardial
ischemia41 or the intracoronary
administration of adenosine42 in dogs
increases renal sympathetic nerve activity. The increase in renal
sympathetic nerve activity is augmented by dipyridamole
in both cases41 42 and attenuated by
aminophylline.41 These experiments are offered as
proof, albeit indirect, that ischemia acting via
adenosine receptors may stimulate sympathetic afferents that
transmit impulses to the central nervous system, where they are
perceived as pain. However, recent neurographic recordings in
cats have failed to show that adenosine excites the same
sympathetic afferents as are activated by coronary
occlusion.43 Although adenosine excites
sympathetic afferents, which are thought to be the primary mediators of
cardiac pain, exogenously applied adenosine also excites vagal
afferents.14 In a related vein, recent direct
neurographic recordings of cardiac vagal afferents has shown
that myocardial ischemia activates vagal afferents in
rats.44 Although the preponderant evidence at
present suggests that ischemia and exogenous
adenosine activate sympathetic
afferents,40 this question has not been
conclusively resolved.
Our present experiments and our previous work suggest that
paradoxic bradycardia is mediated by vagal
afferents,6 7 but a role for sympathetic
afferents has not been completely dismissed, because vagotomy and
sympathectomy both block the
reaction.6 7 By recording vagal and
sympathetic afferents, future experiments will identify conclusively
through which afferents adenosine is working.
Limitations of the Study
Attempting to further evaluate the role of the
A1 receptor in mediating paradoxic bradycardia,
we performed experiments (not included in the present results) with
cyclopentyladenosine (CPA),19 20 33 a
selective A1 receptor agonist. Consistent
with the hypothesis that A1 receptors mediate
paradoxic bradycardia, CPA produced a dose-dependent R-R prolongation
that was associated with a proportionately diminished R-R prolongation
in response to inferior vena cava occlusion. Against this
hypothesis, CPA alone did not induce paradoxic bradycardia during
inferior vena cava occlusion, and it did not shift the dose
of isoproterenol needed to induce paradoxic bradycardia during
inferior vena cava occlusion. At higher doses of CPA and
isoproterenol together, the paradoxic bradycardia response to
inferior vena cava occlusion was extremely prolonged, often
lasting 5 minutes after release of the inferior vena cava
occlusion. This contrasts with other experiments involving
isoproterenol and inferior vena cava occlusion, in which
paradoxic bradycardia disappears promptly after the release of the
inferior vena cava.6 7 8 9 24 29
Despite the failure of CPA by itself to produce paradoxic bradycardia
during inferior vena cava occlusion or to shift the dose of
isoproterenol needed to cause paradoxic bradycardia during
inferior vena cava occlusion, we concluded that
A1 receptors were probably the basis for the
reflex, because DPCPX, a selective A1
antagonist, blocked the reaction. The different conclusions
from the CPA and DPCPX experiments may reside in a difference between
exogenous adenosine (CPA) and endogenously released
adenosine, as reported by other
investigators.27 Further experiments are needed
to address the difference between exogenous A1
agonist (CPA) and endogenous adenosine released in
our experimental model.
These experiments were carried out during anesthesia, which
may alter reflex activity. Future experiments in awake animals are
needed to address this issue.
Conclusions
A reduced cardiac volume combined with
ß1-adrenergic stimulation (isoproterenol)
stimulates paradoxic bradycardia in the rat. Augmenting extracellular
adenosine by dipyridamole markedly increased
the bradycardia-isoproterenol dose response. Blocking the
adenosine receptor by aminophylline, a nonselective, or DPCPX,
a selective A1 antagonist inhibited
the reaction.
Therefore, drugs that increase adenosine concentration or block
the adenosine receptors may increase or inhibit paradoxic
bradycardia, respectively. These findings may have relevance to other
animal models of vasodepressor reaction.
|
Acknowledgments
|
|---|
This study was supported in part by grants-in-aid from the Heart
and Stroke Foundation of Ontario.
|
Footnotes
|
|---|
Reprint requests to Menashe B. Waxman, MD, FRCP(C), Toronto Hospital, General Division, Gerrard Wing, PMCC 3–558, 200 Elizabeth St, Toronto, Ontario, Canada M5G 2C4.
Received December 8, 1997;
revision received March 18, 1998;
accepted April 20, 1998.
|
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Abstract
Introduction
